Getting a new HKS downpipe and ordered studs in advance of them likely having issues during removal. Do I need to buy the fancy expensive nuts that go with due to heat tolerance or would some cheap copper ones work?

The studs are $6.50 (need 4) whereas the nuts are $8.50 (need 8), figured I could save $60 if I didn't need them. Not the biggest deal, I'll get em if it's necessary just saving money for other stuff


P.S. I assume the HKS is a good 3" DP to go with? got one for $220.

Thanks in advance!

 

you don't NEED the oem hardware. any flanged stainless bolt will work. you don't really need to replace any of it all honestly. most times it comes out fine. help yourself and soak it in penetrating fluid in advance. the hks is a good downpipe. easy to install and of course high quality. its got an advantage over the usdm designs because of its shape. it allows for straight and easy access to all 4 nuts.

 

Thanks for the insight! I already got the studs from Atkins, so will use those.

I hope nothing breaks, but my DP has been on the car for 5+ years and I have a history of breaking bolts so I figured I'd be safe.

I plan to reuse the gasket as well, since those seem to be pretty beefy.

 

You might want to consider getting the DP ceramic coated. I did my HKS around ten years ago with jet-hot. It’s held up without issue.

regarding the studs, I got the OEM ones along with a new gasket

 

i like the OE studs, but for the nuts, i think the copper clad ones are nice. they aren't as locking as the factory nuts, but they will come off without breaking things, which for a downpipe is nice.
Mercedes part number is 000-990-31-50, they are unbelievably cheap too

 

https://belmetric.com/nuts/copper-exhaust-nut/


I always recommend biting the bullet for high temp NICKEL anti-seize (not the cheap copper junk at the auto parts store) on all threaded joints regardless. It will greatly reduce thread galling, seizing, and breakage long term.

.

 

When I installed my HKS downpipe, Ray sold me some Mazda studs from some other model that were both OE and cheaper than FD studs. Give Ray a call.

 

maybe RX8 studs? But I thought they were the same.
.

 

or maybe something from a newer car? Mazda kind of defaults to M10x1.50 for the exhaust...

 

I cannot remember the model, but it was not an RX-8. Ray would be able to tell you. I have had no issues since installation, and much cheaper than the "RX-7" studs.

 

Ray advised the inconel FD studs, but the one for the manifold to turbo, instead of the turbo to dp PN. They're slightly shorter making it easier to get bigger downpipes on.

 

Thanks all for the advice!

Now for me to find a damn DP. I ordered one through Kami speed and my delivery date went from 2 weeks to shipping in late July...

Looks like Bonzai might have them, although $110 more expensive.

 

The best easy to get downpipe is the HKS. Fits great, flows great, super well built, and reasonably priced. There are other more exotic ones out there that will perform better but are more costly, like the dual-tube or very large diameter ones. They are also very hard to track down.

With any downpipe, I HIGHLY recommend having a second O2 bung welded at the end of the downpipe for a wideband then having it high temp ceramic coated by Jet-Hot or Calico Coatings. Makes a huge difference in heat under the hood.

Dale

 

Thanks for the advice Dale! I wonder if anyone sells a DP with the bung already, or offers coating? I'm willing to spend a little more for a good one, as opposed to having to modify. I see a few after googling a bit:

Rotary Performance

Petit Racing - has the bung for the sensor already, and they offer ceramic coating for additional $$. Equivalent to Jet hot, or better to just get one in SS and get it coated afterwards?

Bonez - Never heard of this brand in my years of modifying cars, but since it's on rx7.com I assume it's legit?

And of course there's the HKS...


I know I've strayed from the original topic of this post, but figured asking follow-up questions here is better than another post about downpipes. I already have the Racing Beat catback, so would really like to get a quiet exhaust setup with the car as cool as possible.

 

Assuming it's a lhd car, I'd probably go for something other than the HKS.

I must live in another universe, every removal of twins in past days always had one or more of the nuts or studs cactus, even when anti-seize had been applied - inconel absolutely loves to gall.

 

I have the ceramic coated petit, definitely was an improvement over my dented no-name downpipe in spool, tone, and heat. You will have to line it up then install the studs though. The curve is too wide and hits the frame if you try and put it over top of even the shorter studs.

I run petit dp -> no name hi-flow cat->RB twin catback, and it's quite bassy and fairly quiet. Probably gonna go to a Greddy RS-Ti whenever those became available for a bit more noise.

 

+1 on the HKS, or any of the Japanese-style downpipes. The flow difference is negligible and they fit so much better.

 

Do you have any data to back that statement up?

I'd go for a proper LHD-friendly downpipe that takes advantage of the amount of space available with a proper bend right after the turbine housing.

Txfuncars, Bönez is RP/RX7.com's house brand and they've been solid parts for decades now.

 

I bought the HKS DP after fighting enough with a no name pipe that came with an engine I bought. HKS fit up with absolutely no issues!